Fiber optic communications generally employ a modulated light source, such as a light emitting diode (LED), a photodiode (PD) light detector and a glass or plastic fiber interconnecting the LED and PD. In most instances, the LED is modulated by a two-level digital signal and emits one of two light intensities depending on which of the digital signals is applied to the LED. The PD responds to the light intensity conducted by the fiber and provides an electric signal output corresponding thereto, thus, reproducing in a usable electric format the information modulated onto the LED at some remote location.
At this time PN and PIN photodiodes are available. Neither of the prior art structures is entirely suitable since neither can be easily integrated with the receiver circuit in a two-dimensional monolithic chip. A common PIN photodiode available in the prior art is illustrated in FIG. 1. This planar arrangement, in addition to its difficulty with integration, has other substantial limitations and drawbacks.
The PIN photodiode of FIG. 1 is built on a silicon chip 10 which has an I region 11, a thin P region 12, and an N region 13. A ring-like anode 14 is in contact with the thin P region 12 and a silicon oxide layer 15. An aluminum cathode 16 is deposited on the planar surface of the N region 13.
Incident light, for example, light exiting a transmission fiber, passes through the thin P region and is detected by absorption in the I region. However, some of the light passes through, especially at the longer wave lengths due to the limited thickness of the I region in the vertical direction as viewed in the drawing. This dimension must be limited in order to maintain speed since the generated carriers have to travel through the longer distances if this dimension is extended to increase response (see FIG. 4 which shows the effect on transit time with variation of I region thickness. The response curve shown in FIG. 2 illustrates a drop in response above 800 nm caused by the finite vertical dimension of the I region. The graph in FIG. 3 shows the efficiency as a function of I region thickness and wave length, again graphically illustrating the inherent constraints placed on operation by the structure of FIG. 1.
Side entry PIN photodiodes have received little attention and are not at this time commercially available. Optimum structure and characteristcs of this type photodiode have not been considered but are part of the subject matter of this invention and are discussed below.
In addition to the above region thickness tradeoffs, the thin P layer region required to pass the photo energy is electrically undesirable since its thinness increases the resistivity of the device.
The invention contemplates a novel PIN photodiode comprising a semiconductor chip having a thick planar P region separated from a planar N region by a thin planar I region provided with means for admitting photo energy directly to the I region in a direction parallel to the planar orientation of the I region and electric conducting means contacting the P and N regions for connecting the PIN photodiode to electronic circuits formed on the same or other semiconductor chip.